Chlorinated polyvinyl chloride (CPVC) is a rigid thermoplastic material that was introduced in 1960 for water distribution piping that can operate at 100 psi and 180.degree. F. These conditions are safely above typical temperatures and pressures of domestic water heaters. Because CPVC retains physical properties at temperatures well above other thermoplastics, it has been specified for a variety of other extruded and custom-molded products. In view of its inertness in the presence of many corrosive fluids, CPVC has been used widely in electroplating, metal finishing, in other corrosive fluid systems, in pipe and fittings, in pumps, tanks and other products.
The post-chlorinated polyvinyl chloride resins can be processed and formed by conventional techniques such as milling, calendering, extruding, laminating, compression molding, and transfer molding. One major disadvantage that is experienced with CPVC resins is their poor processability. This is exemplified by milling CPVC on a roll mill which results in high torque and high temperatures accompanied by its decomposition. Softening additives or plasticizers have been added to CPVC in order to improve its processability, however, although its processability is thus improved, these additives produce undesirable effects. Some of the more significant detrimental effects produced by inclusion of the additives are heat sensitivity, softness and weakness in terms of lower tensile strength and less desirable chemical and electrical properties than exhibited by CPVC alone. These negative attributes of the additives on CPVC limit usefulness of the modified CPVC in the manufacture of rigid plastic articles.
Poor processability of CPVC has been recognized and attempts have been made to ameliorate this problem. U.S. Pat. No. 3,268,626 to Jennings describes a thermoplastic composition which has substantially the same properties as CPVC resin but which also possesses improved processability and shock resistance. This composition comprises CPVC resin containing small amounts of two copolymers, i.e., a first copolymer of styrene and acrylonitrile containing 20 to 35% by weight of polymerized acrylonitrile units having a dilute solution viscosity of 0.3 to 0.7; and a second rubbery, cross-linked copolymer of butadiene and acrylonitrile containing 20 to 30% by weight of polymerized acrylonitrile units. On the basis of 100 parts by weight of CPVC, amount of styrene-acrylonitrile copolymer can vary from 1 to 15 parts by weight and amount of butadiene-acrylonitrile copolymer can vary from 3 to 15 parts by weight.
U.S. Pat. No. 3,678,132 to Isogawa discloses thermoplastic compositions which have the combination of good molding property, high impact resistance and high heat resistance. The composition is prepared by blending polyvinyl chloride with a terpolymer and a graft copolymer. The terpolymer is prepared by copolymerizing 30 to 80% by weight alpha methylstyrene, 5 to 50% methyl methacrylate, and 3 to 30% acrylonitrile whereas the graft copolymer is prepared by graft copolymerizing onto 15 to 35% of a butadiene polymer, 35 to 65% of a monomer mixture of 50 to 80% styrene, 20 to 50% methyl methacrylate, and 0 to 30% acrylonitrile. Relative proportion of components is 20 to 60% PVC resin, and 80 to 40 of the terpolymer and the graft copolymer. As between the latter two, relative proportion is 40 to 80% of the terpolymer to 60 to 20% of the graft copolymer.
Since PVC processes easily and CPVC does not, and since CPVC has high heat resistance but PVC does not, it should be apparent that CPVC and PVC are different materials and that PVC prior art is not analogous to the patentability issues relating to CPVC.